During the late sixties and early seventies the United States, plagued by a rapid deterioration in its water resources, embarked on a program of wide scale improvement of municipal wastewater treatment. That program was designed to maintain the quality of water in rivers and streams and to halt further pollution of those water resources so that they could be enjoyed by generations to come. As part of that program the Environmental Protection Agency (EPA) assisted municipalities by providing them with billions of dollars of aid to improve the technology of municipal wastewater treatment. This aid greatly improved the state of wastewater treatment throughout the United States to the point where many rivers and streams are now undergoing a renaissance in aquatic life and related environments as the quality of the water in the rivers and streams improves. Despite these efforts, a large number of municipal and private wastewater treatment plants still fail to meet EPA pollution discharge permit limits thereby incurring millions of dollars in fines each year. More important, however, is the fact that wastewater pollution continues. For example in the U.S. alone approximately 4.2 million gallons of effluent from septic tanks is released into the ground every minute. This constitutes a significant source of groundwater pollution and disease. Further, this problem is not just in the U.S. but is of worldwide proportions. Additionally, traditional methods of wastewater treatment result in significant amounts of sludge, which must be disposed of by such expensive means as sludge digestors or by disposal in solid waste landfills or land applications, etc. As will be seen from the description below, the present invention solves these problems.
While more wastewater treatment plants are now in operation as a result of government aid, the processes by which municipal wastewater is treated have not been modified significantly in more than 70 years. Thus while many versions of the original processes are in use, they are all generally similar.
The activated sludge treatment process is employed in more than 80% of all treatment plants in the field of home, municipal and biodegradable industry wastewater treatment. That process involves the use of biological agents, free in solution, to digest organic matter that is dissolved or held in suspension in wastewater.
This activated sludge process has several disadvantages. The efficiency of treatment varies widely during the course of any treatment process giving widely varying pollution results. Further, the activated sludge process is temperature sensitive and does not work well in lower temperatures or during periods of rapid temperature change. Additionally, this method results in a large volume of solid waste sludge, which is a significant disposal problem. Consequently, additional land is used for disposal of the sludge thereby increasing the environmental impact of the treatment of municipal wastewater. In the alternative, expensive sludge digestors would have to be employed at tremendous cost to the construction of wastewater treatment plants.
Activated sludge processes focus on settling out as much of the solids as possible while treating the wastewater with biological agents (bacteria) which digest organic matter. "Sludge bulking", which is the most common source of poor solids separation and therefore poor wastewater treatment, occurs when biological solids do not settle rapidly or compact well. (See Rittman, 1987, page 132). This is thought to be due to the growth of filamentous microorganisms which increases resistance to settling and prevent flocculation. Thus smaller particles are a hinderance to rapid settling, and the flocculation of smaller particles into larger ones is important. Also, the time it takes for settling is important, due to the volume of the influent waste water. Good settling means that larger particles are less likely to be carried into subsequent processing steps or tanks.
Wastewater that is being treated in an activated sludge treatment system has a concentration of organic and inorganic matter suspended in the wastewater. This is referred to as mixed liquor suspended solids, or "MLSS". The suspended particles and dissolved organics from the influent wastewater are mixed together in the MLSS. The amount of oxygen required for aerobic bacteria to completely treat the organic matter in the wastewater is referred to as the biochemical oxygen demand or BOD. The quality of wastewater that is released after treatment is measured in part by the amount of suspended solids or "SS" present in solution, usually in milligrams per liter and the level of BOD of the effluent. The EPA has standards for BOD and SS discharge from waste water treatment plants.
In order to remove and lower the SS and BOD, a process which uses biological treatment was developed. This process, known under various names such as attached growth, fixed biofilm, contact oxidation process, has been used for many years to treat wastewater (for review, see Rittman 1987). Biofilm refers to a layer of biologically active organisms or agents which grow on some form of support media and which digest and otherwise breaks down organic matter suspended or dissolved in wastewater. These biological agents grow to the point where they form a layer or "biofilm" over support materials which in turn provide a greater surface area over which those biological agents can grow and operate. The biofilm is spontaneously "inoculated" from microorganisms present in the influent stream and in the air, and, in the conventional biofilm process grows over a period of weeks during the initial start-up of the system.
The biofilm process has been used in a variety of ways. The process has been used in trickling filters, where the biological agents coat small stones and the wastewater is trickled down through the stones; rotating biological contactors where the biological agents coat a moving mechanical support; a filtration sock in an aeration tank in which biological agents grow in the filtration material; and in floating balls in an aeration tank which also have coatings of biological agents.
These techniques offer a number of disadvantages. For example, the filtration sock has frequent clogging and replacement problems which cause this technology to be fairly impractical and expensive. Biofilm coated floating balls have the disadvantage that the biofilm on the surface of the balls cannot be maintained at the required thickness to facilitate optimum digestion of the organic matter and SS. Further, sludge continues to be produced in significant amounts and must be disposed of by other expensive means such as sludge digestion tanks, dewatering process and discarded in land fills and by land application.
As a result of these problems, a new technology of biological contact oxidation was developed. This process utilizes plastic tubes which are placed in an aeration tank and a biofilm is grown on the inside and outside of the tube media..sup.1 This process therefore combines the activated sludge and biofilm processes. When the BOD concentration is high, greater surface area is required on which biofilm can grow. This requires a smaller tube diameter. However, tube clogging increases as tube diameter decreases; this becomes a significant problem. Thus there is a practical limit to how great a surface area is available for biofilm growth. Tube clogging is further exacerbated when compressed air or surface aeration is used as the aeration source; an increase of biofilm growth causes more rapid tube clogging. FNT .sup.1 The biofilm is grown from the microorganisms present in the influent stream and in the air.
When tube clogging occurs, the efficiency of the system decreases, until the waste treatment facility must be shut down for cleaning. Once cleaning has occurred, and the system is restarted, there is a lag time of efficient wastewater treatment due to the slow formation of the biofilm. It may take as long as three weeks for a wastewater treatment system to be back in full operation.
Biological contact oxidation technologies utilize a slow circulatory flow and gentle mixing process. Slow circulation and gentle mixing has been used for several reasons. First, slow circulation resulting from gentle mixing is thought to result in greater biological degradation. Second, several types of filamentous microorganisms, which contribute to sludge bulking, have a high affinity for dissolved oxygen. Third, slow circulation in the treatment chamber or compartment discourages solid particle breakdown; this facilitates the settling out of the large particles to remove the particles from the liquor, lowering the SS concentration in the effluent.
Thus the generation of MLSS particles in the conventional processes previously described is generally thought to be undesirable. See Rittmam (1987). While the use of biofilm support structure does increase the surface area and therefore the amount of biological agents able to digest organic matter, the teachings in the art dictate that for a vast majority of treatment processes a gentle fluid flow and larger particle sizes of organic solids are required; both of which cause clogging of the biofilm support structure in use and a significant amount sludge of which must be disposed of by expensive means.
An additional problem with current treatment processes is that dissolved oxygen ("DO") is usually too low in the effluent that is released in the environment or goes on to tertiary treatment. Generally, any aeration of the wastewater does not result in DO concentrations in that treated wastewater sufficient to satisfy EPA requirements. DO is necessary not only for the maintenance of many forms of aquatic life but for other forms of subsequent treatment (re-order). This means that either a re-aeration process must be employed thereby adding time and expense to the treatment process, or else partially treated water is dumped into rivers and streams with a consequent adverse effect on life in those aquatic environments unless expensive tertiary treatment is used. In most locales, the pollution discharge permits issued by EPA require certain DO levels that are not now being met through use of most of the existing technology.